A piezo injector of this type comprises a piezoelectric actuator that converts an electrical control signal into a mechanical stroke movement. A nozzle needle is controlled by means of this stroke movement and it is possible using said nozzle needle to release through the injection holes of a nozzle unit the quantity of fuel flow more or less required in order to be able to inject in an appropriate manner into a cylinder of the motor vehicle a desired quantity of fuel that is dependent upon the electrical control signal.
Fuel injection systems of this type contribute greatly to the demanding wishes of customers being fulfilled and to the legal requirements with respect to fuel consumption and toxic emissions of the motor vehicle being fulfilled. This applies in particular to auto-ignition combustion engines having piezo-pump-nozzle systems and to piezo-common-rail systems.
Error indications, for example fuel leakages, sticking valves, deposits, leakage currents, etc., that occur in these systems generally result in a vehicle behaving in a manner that is undesirable, such as loss of power, increased toxic emissions or else also in an error memory lamp being activated. These error indications can occur both in the hydraulic system and also in the electrical system.
First and foremost, when using on-board diagnostic strategies in the dynamic operation of a vehicle, there is a limit as to how close the cause of errors in the injection system can be defined, let alone being able to ascertain precisely such causes, without during the course of the diagnosis having a negative influence on the manner in which the system behaves. In addition, the manufacturer of the motor vehicle frequently does not wish intrusive tests to be performed during the vehicle operation. Furthermore, the extent to which the location of the respective cause of the error can be ascertained is limited as a result of the limited amount of sensor information available on board.
Moreover, particularly moderate error indications in an injection system only influence the driving behavior in dependence upon the operating point. For example, a relatively high-ohm leakage resistance between the electrical connection of the piezoelectric actuator and the electrical ground has only a slight influence on short fuel injection operations and in fact is dependent upon the time constant that is obtained from the value of the leakage resistance and the capacity of the piezo element. In addition, the extent of the influence is still compensated by the system in dependence upon the value of the short circuit resistance and upon the actual operating point, for example depending upon whether the prevailing rotational speed or loading is in the low or middle range. This can be achieved, for example, by providing greater control energy for the piezoelectric actuator.
Moderate error indications only influence the manner in which the system behaves if it is necessary to provide a comparatively large fuel flow for the prevailing operating mode of the motor vehicle, in other words to provide a comparatively long period of control. In such cases, any loss of charge of the piezo actuator can over time result in an undesired reduction of the injection rate and consequently in a reduction of the quantity of fuel being injected. This reduction of the quantity of fuel being injected causes a loss of power that in many cases is associated with an increased exhaust emission.
Error indications of this type cannot be reproduced in a workshop or can only be reproduced at great expense, for example using a power-absorption roller and/or additional sensors, and it consequently represents a great challenge in a workshop when searching for errors.
Components that are still functional are frequently replaced unnecessarily in a workshop owing to a lack of precise knowledge of the cause of a prevailing error. Also, frequently too many components are replaced. For example, a still functional control unit (ECU) or an entire injector set is unnecessarily replaced although a prevailing undesired behavior of the system had been caused, for example, by a single defective injector or by a contaminated male connector in the cable harness.
Furthermore, manual interventions in the injection system of a motor vehicle frequently results undesirably in contaminants being introduced into the injection system and as a result components being damaged.
In addition, unless an initially moderate error is discovered, it can become a major error during the course of time. The consequence of a major error of this type is in many cases a total failure of the injection system and consequently the respective motor vehicle comes to a standstill.
An additional problem is that legal requirements for monitoring the functions of a motor vehicle have recently become more stringent. This applies both for the automotive market in Europe and also in the USA. It was previously sufficient to recognize and indicate serious errors in the system, for example, short circuits to the electrical ground of the motor vehicle. The fundamental tenor of current legislation is on the other hand the requirement to recognize any error that affects in any way the exhaust gas emission of the motor vehicle. This also includes recognizing the above mentioned moderate errors.
DE 10 2006 036 567 B4 discloses a method for ascertaining the functioning condition of a piezo injection of a combustion engine, in which the input variables of a control circuit for injecting fuel are the voltage value and the charge value. Furthermore, the continued capacity progression for the measured piezo injector is calculated based on a new capacity and the last stored capacity values with the aid of a mathematical approximation method. An actual malfunction of the piezo injector is recognized by virtue of the fact that a measured capacity value is outside a first upper and lower tolerance range by the calculated capacity progression. The piezo injection is immediately switched off if the measured capacity value is outside a second upper and lower threshold range by the calculated capacity progression, wherein the threshold range includes the tolerance range.
DE 103 36 639 A1 discloses a method and a device for diagnosing the function of a piezo actuator of a fuel measuring system of an internal combustion engine. The piezo actuator is charged using a pre-determinable electrical voltage and the charge quantity available in the case of this voltage is compared with a desired charge quantity that is to be expected in the case of this voltage. The functionality of the piezo actuator is ascertained from the difference between said charge quantities.